Damping of high-energy particle-hole-type nuclear excitations: A semimicroscopic model

M H Urin

doi:10.1103/physrevc.87.044330

Abstract

A semimicroscopic model (particle-hole dispersive optical model) is formulated to describe the main relaxation modes of high-energy particle-hole-type excitations in medium-heavy mass nuclei. Within this model, Landau damping and the single-particle continuum are considered microscopically, while the spreading effect is treated phenomenologically, employing a statistical assumption. Description of direct nucleon decay of the above-mentioned excitations (including giant resonances) is a unique feature of the proposed model, which in applying to closed-shell nuclei is arranged for practical implementations. The methodical similarity of formulations of the single-quasiparticle and particle-hole optical models is emphasized.

Highlights

A great variety of high-energy particle-hole-type excitations, including giant resonances (GRs), is characterized by three main relaxation modes. They are: (i) the particlehole (p-h) strength distribution, or Landau damping, which is a result of the shell structure of nuclei; (ii) coupling of (p-h)-type states to the single-particle (s. p.) continuum that leads to direct nucleon decay and related phenomena; (iii) coupling of (p-h)-type states to many-quasi-particle configurations, or chaotic states, that leads to the spreading effect
As applied to description of GR damping, we developed a semi-microscopic approach based on the continuumRPA versions of the Migdal’s finite Fermi-system theory [1]
Within this approach Landau damping and coupling to the s. p. continuum are described microscopically, using a mean field and p-h interaction, while the spreading effect is phenomenologically taken into account directly in the cRPA equations for energy-averaged quantities in terms of the imaginary part of an effective s. p. opticalmodel potential

Summary

Introduction

A great variety of high-energy particle-hole-type excitations, including giant resonances (GRs), is characterized by three main relaxation modes. The intensity of the imaginary part, which is parameterized as a universal function, exhibiting the saturation-like energy dependence, is adjusted to describe the experimental GR strength distribution In applying to such a description the semi-microscopic approach is intermediate between “fully microscopic”. To extend the above-described approach on arbitrary (but high enough) excitation energies and to verify validity of this approach in the energy region of a given GR, we formulate in brief terms a new semi-microscopic model without using additional model parameters We call this model, as the particle-hole dispersive optical model (PHDOM), in view of a methodical similarity with formulation of the well-known single-quasiparticle dispersive optical model [4]. Preliminary considerations of the PHDOM are given in [3, 5]

Generalization of the cRPA standard version

Generalization of the cRPA non-standard version

Conclusive remarks